Heretofore in the field of image focusing devices and techniques, it has generally been required that several complex and expensive photodetectors be utilized in combination with beam splitters so as to form several images from a single projected image and make comparisons of the several images to achieve proper focusing. Such apparatus and techniques are shown in U.S. Pat. No. 3,450,883.
It has further been known that a photoconductive element may be utilized in an automatic focusing system to control the focusing adjustment thereof as a function of the conductivity of the photoconductive element; such conductivity being a function of image clarity. The characteristic relationship between the conductivity of a photoconductive element and image clarity is well known in the art and clearly related in U.S. Pat. No. 3,562,785. However, this patent again incorporates a complex electromechanical structure to achieve the focusing technique while utilizing the benefits of the relationship between conductivity and focus.
Consequently, it is an object of the present invention to present a technique and structure for achieving the automatic focusing of optical images wherein a single inexpensive detector is necessitated.
Yet another object of the invention is to present an automatic focusing device wherein a single simple rotating disk utilized in conjunction with a minimum amount of electronic circuitry achieves the focusing technique.
Still a further object of the invention is to present such an automatic focusing device having short response times to change its focus conditions.
A further object of the invention is to present an automatic focusing device wherein the output of the device can easily be adapted to many optical focusing applications.
Yet another object of the invention is to present an automatic focusing device which is simplistic in design, inexpensive to construct, reliable in operation, and conducive to construction with state of the art elements.
These objects and other objects which will become apparent as the detailed description proceeds are achieved by an image projection system having a light source for projecting an image bearing beam upon an image plane, the improvement of an automatic focusing device, comprising a photoconductor receiving a portion of the light beam and creating signals indicative of image clarity; a focus delay plate interposed between the projection system and the photodetector and altering the distribution of the light incident to the photoconductor in a predetermined manner; and circuit means connected to the photoconductor and responsive to the signals of the photoconductor to create error signals indicative of the degree of focus of the light beam image.